[ABA2015]

hi all
what does flash point exactly means? for example for a solvent like pyridine flash point is 17^oC which it will evaporate and forms an ignition mixture in air but the boiling point is 115^oC so if it will make an explosive mixture with air in 17 so how it is used as a solvent and for reflux?

[discodermolide]

The flash point is the lowest temperature at which a liquid or solid gives off enough vapour to form a flammable air-vapour mixture near its surface. The lower the flash point, the greater the fire hazard.

[ABA2015]

thus when the flash point is very lower than bp so how we can use that solvent for reflux or temperatures near bp?

[discodermolide]

You use it carefully!
A certain concentration of vapor in the air is necessary to sustain combustion, and that concentration is different for each flammable liquid. The flash point of a flammable liquid is the lowest temperature at which there will be enough flammable vapor to ignite when an ignition source is applied.
So you keep the vapour concentration low by using a condenser, which does not allow the vapour to reach the flash point concentration.

[Arkcon]

You could also try to keep the reflux under an inert atmosphere.

[DrCMS]

So you keep the vapour concentration low by using a condenser, which does not allow the vapour to reach the flash point concentration.

I think you'll find that for pretty much all commonly used organic solvents that statement is untrue.

Solvents such as acetone, toluene, hexane, THF, diethyl ether have flash points below typical lab temperatures.

To use these solvents safely in the lab you should work under an inert gas and remove as many possible ignition sources as possible.

[curiouscat]

So you keep the vapour concentration low by using a condenser, which does not allow the vapour to reach the flash point concentration.

I think you'll find that for pretty much all commonly used organic solvents that statement is untrue.

Solvents such as acetone, toluene, hexane, THF, diethyl ether have flash points below typical lab temperatures.

Sure. But what you say is sort of orthogonal to what @disco wrote. He didn't mention temperature did he?

He's saying keep the concentration outside the flammability envelope. If I'm reading it correctly. The solvent conc. inside a closed system is often too rich to ignite even if there's a tiny bit of air.

If you didn't have a condenser you'd release vapors outside system. There they won't be rich enough to not ignite.

[DrCMS]

A condenser does not keep the solvent concentration below the LEL. 

At typical room temperatures the vapour concentration above most commonly used solvent is above the LEL.

In a closed system the vapour concentration maybe above the UEL at reflux but that is a poor argument to say it's safe to reflux flammable solvents without any other precautions.  As you heat up or cool down the system the vapour concentration will be below the UEL but still above the LEL and there will be a flammable atmosphere inside.

[curiouscat]

that is a poor argument to say it's safe to reflux flammable solvents without any other precautions.

Agree. To me it's a two pronged strategy:

(1) Stay outside flammability envelope (2) Avoid sources of ignition just in case (1) fails.

Inerting & cold condensers achieve (1) within & outside the system.

[ABA2015]

sorry but a funny question :-[there is some bottles of these solvents or chemical substances in lab and in the temperature of flash point such as pyridine in 17 oC and they are not protected with inert gas or any other  way to be cooled why they don't explode in lab temperature? is it because they are not in the range of between LEL and UEL? how it happens?

[curiouscat]

sorry but a funny question :-[there is some bottles of these solvents or chemical substances in lab and in the temperature of flash point such as pyridine in 17 oC and they are not protected with inert gas or any other  way to be cooled why they don't explode in lab temperature? is it because they are not in the range of between LEL and UEL? how it happens?

You take a flame near the mouth of such a bottle and bad things are likely to happen.

[Arkcon]

Chemicals above their flash point still need an ignition source to ignite.  We've mentioned this a few times in this thread.  When I took organic chem, we did gentle heating for reflux or distillation in a fume hood with a steam bath.  So there's no risk of ignition when 30 people were all working at once.

You take a risk with each bottle of solvent you order and store in your lab.  If your laboratory safety inspector is competent, they audit your lab from time to time, and may at some point simply tell you -- No, you can't have more bottles of flammables, as dictated by city fire code, for this facility.  And you will have to stop the "Gee, whiz, what can I use next" phase of your procedures and start to plan around legal obligations.  That right, even if you're sure you know what you're doing, and nothing bad has happened in your entire career, the fire department comes up to you and says -- Stop.

[curiouscat]

You take a risk with each bottle of solvent you order and store in your lab.  If your laboratory safety inspector is competent, they audit your lab from time to time, and may at some point simply tell you -- No, you can't have more bottles of flammables, as dictated by city fire code, for this facility.  And you will have to stop the "Gee, whiz, what can I use next" phase of your procedures and start to plan around legal obligations.  That right, even if you're sure you know what you're doing, and nothing bad has happened in your entire career, the fire department comes up to you and says -- Stop.

Back in the days, it used to be a practice at some university labs to store the larger bottles / carboys of solvents in a separate external room. Just in case.